1. Field of the Invention
The present invention relates to a blast-furnace operation method, and more specifically to a method of stably operating a blast furnace without causing the furnace to be out of condition i.e. normal operating parameters, and preventing furnace accident by quickly and properly controlling the thermal balance in the furnace.
2. Description of the Prior Art
Ores and coke are alternately introduced in the form of layers into a blast furnace through the top of the furnace, while the high-temperature air is introduced through tuyeres located at a lower portion of the furnace. The coke in the vicinity of the tuyere burns owing to the high-temperature air being introduced, producing reducing gas (CO) and heat, which rises toward the top of the furnace. The burden materials from the top of the furnace come into contact with the high-temperature reducing gas in a counter-current manner, descend while exchanging the heat and undergoing reduction melt, separate into pig iron and slag in the bottom of the furnace, and accumulate in a hearth.
The reduction reaction of the burden materials proceeds nearly over the whole area of the blast furnace in the direction of its height. However, the mode of the reducing reaction differs in various portions of the furnace, i.e. in a low-temperature zone at a relatively higher portion of the furnace and a high-temperature zone in the lower portion of the furnace there develop characteristic differences in the amount of the heat required for the reactions and in the amount of a reducing agent, i.e. carbon supply such as coke required. In the upper portion of the furnace where temperatures are less than about 1000.degree. C., the iron oxide is reduced through the exothermic reaction represented by the following formula, EQU FeOx+nCO.fwdarw.Fe+xCO.sub.2 +(n-x)CO [I]
This reaction mechanism is called the indirect reduction reaction. To cause this the reaction to proceed efficiently, it is necessary to supply an excess of CO gas so that CO.sub.2, which is a reaction product, is maintained at a value smaller than the value derived from the equilibrium relationship. Usually, n in the above formula [I] must be greater than 3. Therefore, to reduce one mole of FeO to Fe, more than 3 moles of a reducing agent are required.
In the high-temperature zone at the lower portion of the furnace, the two reactions proceed simultaneously as represented by the following formulas, EQU FeO+CO.fwdarw.Fe+CO.sub.2 [II] EQU C+CO.sub.2 .fwdarw.2CO [III]
As presented by the following formula, however, the above reactions apparently acquire an additional mechanism which is direct reduction by the solid carbon. This reaction is called the direct reduction reaction. EQU FeO+C.fwdarw.Fe+CO [IV]
The reduction of the molten FeO with the solid carbon at the lower portion of the furnace is also represented by the formula [IV]. The direct reduction reaction absorbs very great amounts of heat. To cause the reaction to proceed efficiently, therefore, it is necessary to operate supplemental heat. Accordingly, if the direct reduction reaction becomes excessive, fuel is required in amounts greater than that used as a reducing agent, so that the fuel consumption rate (hereinafter fuel rate) is increased.
Thus, the indirect reduction reaction [I] and the direct reduction reaction [IV] in the blast furnace are greatly different from each other in regard to thermal behaviour, and the reaction quantity ratio between the two reactions (hereinafter referred to as "direct reduction ratio") greatly affects the condition of the furnace heat causing the fuel rate to be considerably changed. The fuel rate changes depending upon the direct reduction ratio, e.g. when the direct reduction ratio is adjusted to a predetermined value, the sum of carbon that serves as the reducing agent and carbon that serves as a source of heat becomes minimal, enabling the operation to be carried out at a low fuel rate.
The blast furnace which is stably operated at a low fuel rate represents a state of operation in which the heat consumed in the furnace is neither excessive nor insufficient, and the reduction is efficiently carried out. In other words, the stability and the fuel rate of the blast furnace are strongly affected by the direct reduction ratio. When the direct reduction ratio is too small, the operation is carried out with the furnace being excessively heated; therefore, the furnace condition becomes unstable due to the excess amount of the heat required, and the fuel rate becomes high. Conversely, when the direct reduction ratio is too great, the furnace heat becomes too small. Therefore, the furnace condition becomes unstable, and an increased amount of the fuel is required to supplement the heat, eventually resulting in an increased fuel rate.
The abovementioned unstable furnace conditions cause not only the fuel rate to be increased and the operation efficiency correspondingly decreased, but also invites frequent occurrence of such accidents as overheat or lack of heat, causing the operation to be temporarily interrupted. To prevent such inconvenience, the direct reduction ratio in the furnace must be suitably controlled to maintain a constant heat balance so that the furnace heat does not become excessive or in short supply.
To strictly determine whether the heat in the blast furnace is excessive or in short supply, it is necessary to calculate the heat balance by taking into consideration all of the items related to the heat input and heat output of the blast furnace. Since the calculation is very complicated, a large computer has recently been put into practical use. However, when the direct reduction ratio is either extremely small or great, the reaction in the furnace is in an unsteady state, making it extremely difficult to correctly calculate the heat balance. Consequently, the furnace condition often becomes out of balance, presenting such serious problems as overheat or lack of heat. A great deal of research was required before the blast furnace could be completely controlled and these problems resolved.
In order to solve the abovementioned problems, the inventors have found thre principal factors which cause the heat input and heat output of the furnace to vary, i.e., the reducibility of the charged ores, the rate of the ores to the coke and the oxygen volume in blast. It has been found in accordance with the invention that the heat balance in the furnace can be accurately predicted based upon these three factors and that proper decisions regarding the condition of the furnaces can be rendered relying upon the relations among these factors.